Re: [RFC PATCH v2 04/11] KVM: guest_memfd: Add KVM capability to check if guest_memfd is shared

[David Hildenbrand <david@redhat.com>]

On 29.01.25 18:23, Fuad Tabba wrote:
> Add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which indicates
> that the VM supports shared memory in guest_memfd, or that the
> host can create VMs that support shared memory. Supporting shared
> memory implies that memory can be mapped when shared with the
> host.
> 
> For now, this checks only whether the VM type supports sharing
> guest_memfd backed memory. In the future, it will be expanded to
> check whether the specific memory address is shared with the
> host.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   include/uapi/linux/kvm.h |  1 +
>   virt/kvm/guest_memfd.c   | 13 +++++++++++++
>   virt/kvm/kvm_main.c      |  4 ++++
>   3 files changed, 18 insertions(+)
> 
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 502ea63b5d2e..3ac805c5abf1 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -933,6 +933,7 @@ struct kvm_enable_cap {
>   #define KVM_CAP_PRE_FAULT_MEMORY 236
>   #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
>   #define KVM_CAP_X86_GUEST_MODE 238
> +#define KVM_CAP_GMEM_SHARED_MEM 239
>   
>   struct kvm_irq_routing_irqchip {
>   	__u32 irqchip;
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 86441581c9ae..4e1144ed3446 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -308,6 +308,13 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
>   }
>   
>   #ifdef CONFIG_KVM_GMEM_SHARED_MEM
> +static bool kvm_gmem_is_shared(struct file *file, pgoff_t pgoff)

I assume you want to call this something like:

kvm_gmem_folio_is_shared
kvm_gmem_offset_is_shared
kvm_gmem_range_is_shared
...

To make it clearer that you are only checking one piece and not the 
whole thing.

But then, I wonder if that could be handled in kvm_gmem_get_folio(),
and e.g., specified via a flag?

kvm_gmem_get_folio(inode, vmf->pgoff, KVM_GMEM_GF_SHARED);

Maybe existing callers would want to pass KVM_GMEM_GF_PRIVATE, and the 
ones that "don't care" don't pas anything?

-- 
Cheers,

David / dhildenb